After a century of intensive use of fossil fuels as the predominant energy source for driving high-tech civilization, these natural resources are running low and alternative fuels must be introduced in order to be able to maintain the fast development of human civilization. Furthermore, the ever-increasing consumption of fossil fuels pollutes the environment and there are claims it even contributes to global warming through the extensive production of greenhouse gases that block heat emission from our planet. It is therefore evident that real-life alternative energy sources must be developed in order to replace traditional ones. These should be inexpensive, safe, non-polluting and “user friendly” so as not to impede the development of dynamic human society.
Containing the highest energy density per unit mass and producing only water upon combustion, hydrogen is considered as one of the most efficient and environmentally friendly candidates as a future fuel. Hydrogen is a very energetic material compared to conventional fossil fuels and burns in air at a wide range of concentrations (5%-75%). Moreover, in contrast to fossil fuels, the combustion of hydrogen is considered free of pollution, as it generates only water as a by-product.
The concept of “Hydrogen Economy”, involving the use of hydrogen as a general energy carrier, was suggested as early as 1972. However, hydrogen storage became one of the key points to access the attractive “hydrogen age” since then. The low energy density of heavy hydrogen tanks makes most commercial applications of hydrogen unfavorable. Thus, to achieve hydrogen economy, a major challenge is finding suitable hydrogen carriers. For decades, scientists have searched for suitable hydrogen storage materials. Inorganic or metal-organic systems, such as main-group hydrides, metal organic frameworks, metal clusters, and nanostructured materials, have been explored for this purpose. Unfortunately, all of these efforts suffer from significant limitations.
On the other hand, organic compounds received much less attention as hydrogen carriers, because reversible H2 release under reasonable temperatures was not achieved until 2005. Recently, organic compounds, such as formic acid, methanol-water, formaldehyde-water and carbohydrates, were intensively studied as potential hydrogen storage materials. Among them, “liquid organic hydrogen carriers” (LOHC), which can be dehydrogenated and hydrogenated with considerable amounts of hydrogen and might be used for transportation, are of special interest. An attractive LOHC of potential commercial interest has been N-ethylcarbazole, which was first studied by Air Products and Chemicals. Hydrogenation of N-ethylcarbazole to perhydro-N-ethylcarbazole consumes 6 equivalents of H2, resulting in hydrogen storage capacity of as high as 5.8 wt %. However, many disadvantages still exist in this system, including the requirement of high H2 pressure for the hydrogenation step and high reaction temperature for the dehydrogenation step, and the need of different catalysts for these steps. Two other recent examples of LOHCs are 2-methyl-1,2,3,4-tetrahydroquinoline and 2,6-dimethyldecahydro-1,5-naphthyridine, which can be reversibly dehydrogenated to 2-methylquinoline and 2,6-dimethyl-1,5-naphthyridine, respectively, catalyzed by Ir complexes. However, these two systems suffer from high catalyst loading (5 mol %), relatively expensive liquids, and in the case of 2-methyl-1,2,3,4-tetrahydroquinoline, low hydrogen storage capacity.
The goal of the Fuel Cell Technologies Office (FCTO) of the United States is to provide adequate hydrogen storage for onboard light-duty vehicle, for material-handling equipment, and for portable power applications to meet the U.S. Department of Energy (DOE) hydrogen storage targets. By 2020, Fuel Cell Technologies Office (FCTO) of the United States aims to develop and verify onboard automotive hydrogen storage systems achieving targets that will allow hydrogen-fueled vehicle platforms to meet customer performance expectations for range, passenger and cargo space, refueling time, and overall vehicle performance Specific system targets include hydrogen storage capacity of 5.5 wt %.
The inventors of the present invention have previously reported that pyridine-based PNN and PNP ruthenium pincer complexes (i) to (iv) (FIG. 1) efficiently catalyze several C—O and C—N bond forming dehydrogenative coupling reactions, giving pure hydrogen as byproduct, and also catalyze the reverse hydrogenation reactions. For example, by employing the dearomatized PNN catalyst (ii), amides are produced directly from alcohols and amines, with liberation of H2. Complex (ii) can be obtained in situ by deprotonation of complex (i) with a base. The reverse reaction, i.e. hydrogenation of amides to form alcohols and amines, was also achieved under mild hydrogen pressure, using the same catalyst (Scheme 1).

It was further reported that β-aminoalcohols can undergo dehydrogenative coupling to form cyclic dipeptides (diketopiperazines) (Scheme 2a) or oligopeptides (Scheme 2b), depending on the substituent R. Thus, in case of R=Me (2-aminopropan-1-ol), linear peptides were formed.

U.S. Pat. No. 8,178,723, describes methods for preparing amides, by reacting a primary amine and a primary alcohol in the presence of Ruthenium complexes, to generate the amide compound and molecular hydrogen.
U.S. Pat. No. 8,586,742, describes methods for preparing primary amines from alcohols and ammonia in the presence of Ruthenium complexes, to generate the amine and water.
PCT patent publication no. WO 2012/052996, to some of the inventors of the present application, describes methods of using Ruthenium complexes for (1) hydrogenation of amides to alcohols and amines; (2) preparing amides from alcohols and amines; (3) hydrogenation of esters to alcohols; (4) hydrogenation of organic carbonates to alcohols and hydrogenation of carbamates or urea derivatives to alcohols and amines; (5) dehydrogenative coupling of alcohols to esters; (6) dehydrogenation of secondary alcohols to ketones; (7) amidation of esters (i.e., synthesis of amides from esters and amines); (8) acylation of alcohols using esters; (9) coupling of alcohols with water to form carboxylic acids; and (10) dehydrogenation of beta-amino alcohols to form pyrazines.
Clearly, the development of inexpensive and abundant organic compounds with potentially high capacity to store and release hydrogen, ideally using the same catalyst for both loading and unloading hydrogen under relatively mild conditions, is a major challenge with no acceptable solutions known at this time.